Histamine is a powerful vasoactive mediator which is released from specifically sensitized tissue mast cells and basophils following exposure to specific allergens, thus triggering an allergic response in an individual. While in vivo diagnosis of an allergic reaction may be accomplished by an allergen skin test, such testing may be dangerous due to the possibility of an anaphylactic reaction, may sensitize the patient to the allergen and is generally uncomfortable. In vitro test methods are available for measuring the concentration of allergen-specific IgE antibody in serum. However it is believed that a more useful determination would be that of histamine release which may reflect more accurately the in vivo allergy response. Accordingly, the present invention provides histamine derivatives, immunogen conjugates comprising histamine or said histamine derivatives coupled to immunogenic carrier materials and antibodies prepared against such immunogen conjugates. Such antibodies are useful in immunoassays for determining the quantity of histamine in biological or laboratory fluids as, for example, following challenge of sensitized basophils with a battery of allergens.